Application of hot air‐assisted radio frequency as second stage drying method for mango slices

Abstract

This study applied hot air-assisted RF (HA-RF) as second stage drying method for mango slices.Hot-air drying was used as first stage drying to reduce moisture content to about 40% (w.b.), then HA-RF drying was applied as second stage drying to further reduce moisture content to 18% within 45 min with 4.5 cm sample thickness (hot air temperature: 60 C, electrode gap: 7.5 cm). The time for the two-stage drying process was about 5 hr, which was clearly lower than that of hot-air drying (8 hr) or vacuum drying (7 hr). HA-RF as second stage drying maintained sample overall quality very well except for minor vitamin C degradation (retention rate: 91%);Overall quality of mango slices after the two-stage drying was better than that dried by hot-air drying, and close to that dried by vacuum drying. This study demonstrated HA-RF could be used as second stage drying method for mango slices. Practical applicationsThis study applied HA-RF heating as second stage drying method for mango slices, and research findings in this study indicated that HA-RF drying could reduce drying time and improve sample quality. This study proposed a new two-stage drying method for mango slices, and this technology can be easily scale-up for commercial application. And the two-stage drying efficiency and products quality could be further improved by other novel and efficient drying techniques as first stage drying.

Radio Frequency Drying Behavior in Porous Media: A Case Study of Potato Cube with Computer Modeling

“…Results showed hot-air-assisted RF drying reduced 30% of the drying time compared to solely hot air drying and maintained the good quality of carrot slices. Zhang et al [ 8 ] adopted a two-stage drying method to treat mango slices, which utilized hot air to dry mango slices to 40% (w.b.) in the first stage and then used hot-air-assisted RF heating (HA-RF) to continue to dry the product to 18% (w.b.).…”

Characteristics and Quality Analysis of Radio Frequency-Hot Air Combined Segmented Drying of Wolfberry (Lycium barbarum)

“…L * values decreased from 33.84 and 36.06 to 31.01 and 32.16, respectively, and a * values decreased from 30.15 and 31.59 to 28.02 and 26.95, respectively, when the plate spacing (from 80 mm to 100 mm) and vacuum degree (from 0.015 MPa to 0.035 MPa) gradually increased; however, the Δ E increased to 13.89 and 14.35. As the plate spacing and vacuum degree increased, the heating rate of the sample decreased and the environmental humidity in the RF drying differed from hot air drying, resulting in significant color difference changes [ 22 , 23 , 24 , 25 ]. The total color difference Δ E of dried wolfberries increased from 10.53 to 12.71 when the hot air temperature increased from 55 °C to 60 °C, which was significantly higher than natural drying ( p < 0.05), but lower than single hot air drying ( p < 0.05).…”

“…The three dried wolfberry samples under each group of different drying parameters were randomly selected for colorimetric analysis measured by a CIELab system, the reference of the illuminant was D65, SCI, and the degree of the observer was 10°. Three whole samples in each group were measured and color calculated as follows [ 22 , 23 , 24 , 25 ]: where Δ E denotes total chromatic aberration; L , a , and b represent the brightness value, red and green value, and yellow and blue value of fresh wolfberry samples, respectively; and L *, a *, and b * denote the brightness value, red and green value, and yellow and blue value of dried wolfberry products, respectively.…”

“…Each dehydration time was 10 min. Final samples were immersed in 100% tert-butanol for 15 min and then placed in a vacuum freeze drying machine for processing [ 22 , 23 , 24 , 25 , 28 , 29 , 30 ]. The sample observation surface was placed upward and fixed on the scanning electron microscopy sample table with conductive tape after treatment, and the surface was coated with an ion sputter-coating instrument for 90 s, then placed under a scanning electron microscope.…”

“…However, RF drying is limited for materials with a high moisture content that are easy to rupture and experience shrinkage of the outer skin. For example, selecting a certain electrode gap and vacuum degree can achieve higher quality dehydration of kiwifruit [ 22 , 23 ], apple [ 24 ], and mango fruit slices [ 25 ], as their nutrient retention rate and rehydration rate are comparatively high, but they need to be sliced with an appropriate thickness (6–8 mm) in the early stage of drying, thus preventing a low equilibrium temperature leading to excessive preheating and drying times.…”